Aperture vs. Iris

By Jaxson

  • Aperture

    In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.

    An optical system typically has many openings or structures that limit the ray bundles (ray bundles are also known as pencils of light). These structures may be the edge of a lens or mirror, or a ring or other fixture that holds an optical element in place, or may be a special element such as a diaphragm placed in the optical path to limit the light admitted by the system. In general, these structures are called stops, and the aperture stop is the stop that primarily determines the ray cone angle and brightness at the image point.

    In some contexts, especially in photography and astronomy, aperture refers to the diameter of the aperture stop rather than the physical stop or the opening itself. For example, in a telescope, the aperture stop is typically the edges of the objective lens or mirror (or of the mount that holds it). One then speaks of a telescope as having, for example, a 100-centimeter aperture. Note that the aperture stop is not necessarily the smallest stop in the system. Magnification and demagnification by lenses and other elements can cause a relatively large stop to be the aperture stop for the system. In astrophotography, the aperture may be given as a linear measure (for example in inches or mm) or as the dimensionless ratio between that measure and the focal length. In other photography, it is usually given as a ratio.

    Sometimes stops and diaphragms are called apertures, even when they are not the aperture stop of the system.

    The word aperture is also used in other contexts to indicate a system which blocks off light outside a certain region. In astronomy, for example, a photometric aperture around a star usually corresponds to a circular window around the image of a star within which the light intensity is assumed.

  • Aperture (noun)

    An opening, gap, or hole, usually small and narrow

    “an aperture in a wall”

  • Aperture (noun)

    Something which restricts the diameter of the light path through one plane in an optical system.

  • Aperture (noun)

    The diameter of the aperture (in the sense above) which restricts the width of the light path through the whole system. For a telescope, this is the diameter of the objective lens.

    “This telescope has a 100cm aperture.”

  • Aperture (noun)

    The (typically) large-diameter antenna used for receiving and transmitting radio frequency energy containing the data used in communication satellites, especially in the geostationary belt. For a comsat, this is typically a large reflective dish antenna; sometimes called an array.

  • Aperture (noun)

    The maximum angle between the two generatrices.

    “If the generatrix makes an angle θ to the axis, then the aperture is 2θ.”

  • Iris (noun)

    A plant of the genus Iris, common in the northern hemisphere, and generally having attractive blooms (See Iris (plant)).

  • Iris (noun)

    The contractile membrane perforated by the pupil, which adjusts to control the amount of light reaching the retina, and which forms the colored portion of the eye (See Iris (anatomy)).

  • Iris (noun)

    A diaphragm used to regulate the size of a hole, especially as a way of controlling the amount of light reaching a lens.

  • Iris (noun)

    A rainbow, or other colourful refraction of light.

  • Iris (noun)

    A constricted opening in the path inside a waveguide, used to form a resonator.

  • Iris (noun)

    The inner circle of an oscillated color spot.

  • Iris (verb)

    or door}} To open or close in the manner of an iris.


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